i just read the review in stereophile on the new benchmark dac with usb. the reviewer also used a mac mini as a transport and compared the newer dac to the older benchmark dac without usb (also to a bel canto). he stated that he liked the sound of the toslink hookup better than the usb hookup.
if i was looking to purchase a dac and it came with a usb port at no extra charge, why not go for it. if you have to pay extra for the usb port or they have to take away a toslink/rca/balanced port to put the usb in, i wouldn't do it, it might limit you in the future. I also would not make a statement that usb ports are better than toslink because of jitter issues. Simply not true in all cases. You need to listen for yourself to see if you can hear the difference.